Oilfield operations often entail the use of numerous fluid materials such as drilling fluids and fracturing fluids. A drilling fluid or “mud” is a specially designed fluid that is circulated in a wellbore or borehole as the wellbore is being drilled in a subterranean formation to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts. Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation.
A drilling fluid typically comprises water and/or oil or synthetic oil or other synthetic material or synthetic fluid as a base fluid, with solids in suspension. A non-aqueous based drilling fluid typically contains oil or synthetic fluid as a continuous phase and may also contain water dispersed in the continuous phase by emulsification so that there is no distinct layer of water in the fluid. Such dispersed water in oil is generally referred to as an invert emulsion or water-in-oil emulsion.
A number of additives may be included in such drilling fluids and invert emulsions to enhance certain properties of the fluid. Such additives may include, for example, emulsifiers, weighting agents, fluid-loss additives or fluid-loss control agents, viscosifiers or viscosity control agents, and alkali.
The density of the drilling mud is closely maintained in order to control the hydrostatic pressure that the mud exerts at the bottom of the well. If the mud is too light, formation fluids, which are at higher pressures than the hydrostatic pressure developed by the drilling mud, can enter the wellbore and flow uncontrolled to the surface, possibly causing a blowout. If the mud is too heavy, then the hydrostatic pressure exerted at the bottom of the wellbore can reduce the rate at which the drill bit will drill the hole. Additionally, excessive fluid weights can fracture the formation causing serious wellbore failures. In some cases, failure can cause drilling fluid to be lost to the formation, depleting the drilling fluid, leading to under pressurization or well control problem. Thus, the control of the solids content of the drilling fluid is very crucial to the overall efficiency and safe operation of the rig.
In the most common applications, the density of the drilling mud is increased by adding particulate weighting agents, such as barite and hematite. These particles are prone to settling within the drilling mud under the influence of gravity. This settling is known in the industry as “sag” or “barite sag” and is a persistent and potentially serious drilling problem that occurs most prevalently in directional wells drilled with weighted drilling muds.
Sag can occur, for example, when circulation of the fluid is stopped for a period of time, e.g., when the drill string must be tripped from the well, and is caused by the resulting settling or stratification of the fluid whereby “heavy spots” and “light spots” develop. Sag can also involve movement or shifting of these heavy and light fractions, particularly the “heavy spots,” where components such as barite have become concentrated. Sag may not occur throughout an entire well, but its occurrence in even a small section of the well can cause the problems referred to below. Generally, higher temperatures exacerbate sag while higher pressures tend to retard sag.
Sag is not particularly problematic if the well is vertical or near vertical. The magnitude of the problem may be smaller if the well, or the section of the well in question, is nearly horizontal. However, if the well or a section thereof has a relatively high deviation angle (i.e., angle with respect to vertical), but falling well short of 90 degrees, sag problems can become particularly severe. The advent and recent strides in extended reach drilling, which have resulted in relatively highly deviated wells, e.g., wells with deviation angles of 20 degrees or more, has brought sag problems into focus in the drilling industry.
Sag of the weighting agents in a fluid used in oil field operations can cause large density variations that often lead to significant wellbore pressure management problems and potentially, wellbore failure. Additionally, fluid sag can lead to sticking of drill pipe, difficulty in re-initiating and/or maintaining proper circulation of the fluid, possible loss of circulation and disproportionate removal from the well of lighter components of the fluid.
A number of solutions have been proposed for analyzing the sag properties of a fluid. For instance, U.S. Pat. No. 6,584,833 to Jamison et al. (hereinafter “'833 Patent”) discloses a method of determining the settling rate of a fluid and is incorporated by reference herein in its entirety. However, it is desirable to have a reliable method for measuring and/or monitoring the sag of the weighting agents in the field.
While embodiments of this disclosure have been depicted and described and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure.